The result of a greater smoothness of operation in stands is that, in event of irregularities in the installation location (uneven floor) but also in the event of changes in the loads on the stand, moments of force or torques can occur that cause portions of the stand, in particular the stand arm, to move in drifting fashion in the unbraked state. “Drift” is understood to mean lateral pivoting motions (about a rotation axis), or tendencies toward such pivoting motions of the carrier arm, that are undesired by the user.
Three different actions for the reduction of drift are known from the existing art:
1. Alignment of the stand foot. At least three of, for example, four support feet of the stand foot are made adjustable as to height, and a bubble level on the stand foot indicates its horizontal alignment. A device of this kind is implemented, for example, on the FM2 of the Mitaka company. It ensures a precise adjustment capability, but entails the following disadvantages: Because a three-point adjustment is involved, a displacement at one of the three points creates a need for readjustment at the other two adjustment points. Inexperienced persons have a great deal of difficulty making the adjustment in quick and effective fashion.
There are further disadvantages as well:
Firstly, the adjustment to the stand foot is made while stooped over near the floor.
Secondly, a new adjustment must be made after each change of location, and possibly even after each load change on the stand arm, and usually on all three adjustment devices (or all the adjustment devices that are present).
Thirdly, the installation of the adjustment devices forces the operating person to work, when making the adjustments, in the vicinity of the floor, i.e. in a region that is remote from the patient in an operating room and has a lower degree of sterility.
In addition, an extra assistant might be needed in order to make the adjustments, whereas it would be desirable if an adjustment could be made by the surgeon or an operating room nurse him- or herself.
2. The second known variant comprises a brake to increase the rotational friction. With such a brake, the advantage of smooth operation is deliberately abandoned in order to reduce drift. For the user, this disadvantageously results in an increased need for force when moving the carrier arm. For the surgeon, a high level of exertion makes it difficult subsequently to wield a scalpel, or to do other work that requires a steady hand.
3. The third variant has hitherto been disclosed only in the context of ceiling mounts, and has nothing to do with the actual problem of changes in drift characteristics due to an uneven floor or the like, since in the case of a ceiling mount a fixed attachment point is usually provided from the outset, and no change takes place in the location on the ceiling. When the fixed installation point is created, it is of course optimally laid out so that a ceiling mount in principle has no drift. The adjustment in this context corresponds approximately to the leveling of a stand foot on the floor, although no further change in position is made after this adjustment. On the other hand, the loading of a ceiling mount, possibly with differing weights, creates another risk that in turn can generate drift:
Because of the limited stiffness of a conventional horizontal stand arm that is attached to a vertical support, when a further stand arm attached to that horizontal carrier arm is bent about a vertical pivot axis, the weight of the further carrier arm and of the microscope attached to it results in a torsion on the first carrier arm. Conversely, a flexural load on the stand arm results when the stand arm is extended. The purpose and solution in the case of the ceiling mount was to attain a state in which the microscope lies at the same height when the stand arm is both in the bent position and in the extended position. Once this adjustment has been made, in principle it remains unchanged. A change in the position of one of the stand arms does not change the behavior. Changes in location on the ceiling do not occur.
More detailed examinations of known stand assemblages, for example that of the Dräger series (the Dräger company in Germany has brought to market, under the product designation “Movita,” a series of ceiling mounts that are used in intensive medicine and intensive care) or that of the Kreuzer company in Germany, offer no way toward a solution, since with these ceiling mounts, pivoting movability for surgical microscopes with magnifying properties is not paramount.
Greater frictional forces in rotary bearings do, however, keep them from running out of position, so that in known intensive-medicine ceiling mounts, the problem just mentioned does not even occur. What is required specifically in the case of microscope stands, however, is that the device not drift when the brakes are open, but can be moved with particular smoothness. There exist, for example, mouth switches with which the surgeon can, using his or her lips, on the one hand release the brakes and on the other hand reposition the brakes.
In the aforementioned previously published Patent Application EP-A-1067419, FIGS. 5–7 and the relevant portions of the specification disclose a known mechanism of this kind for preventing drift in ceiling mounts. This known drift prevention device acts, if required, in only one plane. Since the one-time, completely leveled attachment point on the ceiling means that lateral irregularities do not occur, there was hitherto also no need to provide further leveling features or drift compensation features on a ceiling mount. In this respect, this teaching known per se also offers those skilled in the art absolutely no stimulus to ensure that an improvement is created in floor stands with regard to the problem mentioned initially, namely that changes in the inclination of the stand foot usually occur after a change in the location of floor stands.